Glasses and glass-ceramics containing rutile fibers

ABSTRACT

This invention is concerned with the production of composite articles comprising a glass or glass-ceramic matrix containing long fibrous single crystals of rutile (TiO2) of very high aspect ratios grown in situ. The glass articles are formed via the heat treatment of a molten batch consisting essentially, by weight on the oxide basis, of about 45-65% B2O3, 0-30 percent Al2O3, 5-30% TiO2, and 3-30% alkaline earth metal oxides and can exhibit greatly improved mechanical strength and toughness when compared with the original, or fiber-free, glass. The glass-ceramic articles are formed by heat treating the fiber-containing glass in a particular manner to cause the development of fine-grained crystals in situ, resulting in a highly-crystalline article containing long fibrous single crystals of rutile exhibiting very high aspect ratios.

[ Aug. 26, 1975 GLASSES AND GLASS-CERAMICS CONTAINING RUTILE FIBERS [75] Inventors: William T. Brydges, III; Dennis W.

Smith, both of Corning, NY.

[73] Assignee: Corning Glass Works, Corning,

[22] Filed: May 22, 1974 [21] Appl. No.: 472,142

[52] US. Cl 106/47 R; 23/300; 23/305;

65/17; 65/33; 106/39.6; 106/39.8; 106/50 [51] Int. Cl C03c 3/14; C030 3/22 [58] Field of Search 106/48, 49, 47 R, 39.6,

OTHER PUBLICATlONS Harari, A. et a1. Preparation of Single Crystal Metal Oxides, Chem. Abstracts, 69 (1968), item 70955p.

l-lirayama, C- Properties of Aluminoborate Glasses of Group 11 Metal Oxides: 1, Glass Formation and Thermal Expansion,- .1. Am. Cer. Soc. 44( 12) (1961 pp. 602606.

Primary Examiner l-lelen M. McCarthy Attorney. Agent. or Firm-Clinton S. James, Jr.; Clarence R. Patty, Jr.

[ 5 7 ABSTRACT This invention is concerned with the production of composite articles comprising a glass or glass-ceramic matrix containing long fibrous single crystals of rutile (TiO of very high aspect ratios grown in situ. The glass articles are formed via the heat treatment of a molten batch consisting essentially, by weighton the oxide basis, of about 45-65% B 0 0-30 percent A1- 0 5-30% TiO and 3-30% alkaline earth metal oxides and can exhibit greatly improved mechanical strength and toughness when compared with the original, or fiber-free, glass. The glass-ceramic articles are formed by heat treating the fiber-containing glass in a particular manner to cause the development of finegrained crystals in situ, resulting in a highly-crystalline article containing long fibrous single crystals of rutile exhibiting very high aspect ratios.

5 Claims, N0 Drawings GLASSES AND GLASS-CERAMICS CONTAINING RUTILE FIBERS There has been considerable research in the past to improve the mechanical strength and/or fracture toughness of glass bodies by means of fibers dispersed within a glass matrix. Previous attempts to reinforce glass bodies with fibers met with little success principally because of poor dispersion of the fibers, coupled with random orientation thereof, and the difficulty in entraining substantial volume fractions of fibers within the glass matrix.

Glass-ceramic articles are produced through the heat treatment of glass articles to cause the growth of crystals in situ. Because the heat treatment is undertaken in a controlled manner, the crystals developed are characteristically fine-grained and randomly oriented. In general, glass-ceramic articles are highly crystalline such that the physical properties exhibited thereby will more closely approximate those of the crystal phase than those of the parent or original glass. A necessary corollary to this feature is the fact that the residual glassy matrix of a glass-ceramic article will have a far different composition from that of the parent glass since the crystal components will have been precipitated therefrom.

The growth of acicular or needle-like crystals in glass-ceramic articles has been recognized. For example, U.S. Pat. No. 2,920,971, the basic patent in the field of glass-ceramics, reports crystals which can exhibit which morphology. However, theresultant products have been disappointing both from the standpoint of the inherent small size of the fibers so developed and the inability to promote significant orientation of the fibers. Therefore, although extensive fiber growth was indeed possible, the small size and random orientation of the fibers has yielded articles wherein the mechanical strength and toughness are not substantially different from those exhibited by the conventional finegrained glass-ceramic article.

The primary objective of the instant invention has been to develop glass and glass-ceramic bodies containing significant numbers of large fibers having high aspect ratios, which fibers can be made to demonstrate substantial single-direction orientation thereby providing a composite body demonstrating greatly improved mechanical strength and toughness, when compared with the fiber-free glass and glass-ceramic.

With respect to glass bodies, this objective can be accomplished by heat treating a molten mass of a glassforming batch of a particular composition in a manner to cause the growth of large rutile (TiO crystals therein. The melt containing the fibers can then be formed into an article of a desired configuration and the fibers caused to orient in a specific direction during the forming step. Glasses operable in the present invention consist essentially, by weight on the oxide basis, of about 45-65% B 0-30% A1 0 5-30% TiO and 3-30% alkaline earth metal oxides (R0). Calcium oxide (CaO) appears to be the most desirable alkaline earth metal oxide in yielding the largest fibers with the highest aspect ratios, although the presence of magnesium oxide (MgO), strontium oxide (SrO), and/or barium oxide (BaO) will likewise lead to useful fiber growth.

The presence of A1 0 is of particular importance in improving the melting and forming characteristics of the glass. as well as enhancing the chemical durability thereof. Therefore, at least about 5% A1 0 will desirably be included in the glass composition to obtain the most stable composite bodies with the most desirable fiber growth, and the base glass composition will preferably consist essentially, by weight on the oxide basis, of about 50-65% B 0 15-25% A1 0 5-20% TiO and 3-15% RO.

Careful control is demanded in heat treating the molten glass. Thus, the melt is subjected to a temperature which not only lies below the TiO; liquidus but which also is sufficiently elevated to obtain the low viscosity necessary to promote the growth of single crystal rutile of the desired long fibrous morphology. Hence, the temperature and viscosity of the liquid glass are in such relation that the rate of crystal development leads to large crystals demonstrating substantial unidirectional growth rather than an undesirable, fine-grained, heterogeneous crystallization. Each glass composition will exhibit an optimum temperature for growth of rutile crystals but, in general, the operable temperatures will range between about llO0l250C.

The amount of rutile crystallization developed is dependent upon at least three factors: (1) the TiO,, concentration in the glass; (2) the overall parent glass composition; and (3) the time alloted for crystallization to proceed. Normally, at least one-half hour will be required even at the highest concentrations of TiO with, customarily, times of an hour or more being utilized to insure greater amounts of crystallization. However, excessively long exposure times, viz., greater than about 4 hours, can result in the growth of randomly oriented, fine-grained devitrification which may interreact unfavorably with the desired large fibers having high aspect ratios. In sum, the most advantageous products will consist solely of long rutile fibers in a glassy matrix.

Optical microscopy and X-ray diffraction analysis have identified the fibers as rutile single crystals with the c-axis along the fiber axis. Fibers up to about 0.5 inch in length were measured with the majority ranging between about 0.1-0.4 inch. Fiber thicknesses generally varied between about 0.00l-0.004 inch, yielding aspect ratios up to about 200:1, with 50l00:l being most common. This characteristic was of vital interest in achieving reinforcement of the matrix glass. When viewed microscopically in cross section, the rutile crystals exhibited a blocky morphology.

The following table reports a group of working examples, expressed in parts by weight on the oxide basis, which are illustrative of the instant invention. Since the sum of these components closely approximates 100, they can, in essence, be deemed as present in weight percentages. The batch ingredients therefor can be any materials, either the oxide or other compound, which, when melted together, will be converted to the desired oxide in the proper proportions. The batch ingredients were blended together in a ball mill to aid in obtaining a homogeneous melt and then run into a platinum crucible. The platinum crucible was placed in an electrically fired furnace operating at about l200-l300C. and the batch melted for four hours, generally with stirring. Thereafter, the crucible was either transferred to a furnace operating at l lOO-l250C. or the temperature within the melting furnace reduced to that range. In either practice, the crucible containing the molten glass was maintained within that temperature domain for about /a-Z hours to cause the growth of crystals. Thereafter, the molten glass was poured onto a steel plate. By applying moderate pressure upon the slab soformed while it is still somewhat viscous, the fibers can be aligned in the direction of stress. The slabs were annealed by being placed in an annealer operating at cantly affected thereby. Likewise, the addition of such powerful fluxes as F, L1 0, Na O, and K will be avoided. Therefore, the total of all such additions will, preferably, be held below about percent.

inclusions being preferably less than about In general, the common glass forming oxides such as SiO and P 0 will be avoided since the solubility of TiO crystals in the consequent melt will be signifiabO t C- 5 Observation of the fiber growth in the molten batch A n g y, the Slab will be permitted to 0001 leads to the conclusion that the genesis thereof is pribelow a empera re at Whleh the gl can be formed marily due to surface nucleation, i.e., the vast bulk of (defolmiltlon pe before P g tfansfel'l'ed to the crystals begins to grow at the interface between the the annealfir- Placmg the Slabs dlr'ectly Into the f melt and the crucible and at the air-melt surface internealer while very hot can, in some instances, result in 10 f T i h ill promote h growth f acicuiar the growth of heterogeneously'shaped devitrificanon crystals perpendicular to the interface when the viscosthe glass p ity and temperature of the melt are proper.

Sb O is present in its conventional capacity of a fin- Inasmuch as the preferred internal microstructure of ing agent. the glass articles consists essentially of long crystalline TABLE B203 52.5 63.9 56.9 59.7 60.4 59.4 58.4 57.4 56.4 48.4 61.0 51.5 A1203 20.0 20.8 18.6 19.5 19.8 19.8 19.7 19.7 19.7 30.4 TiO 10.0 10.9 9.7 10.2 8.3 9.3 10.3 11.3 12.3 9.9 26.9 15.5 C210 60 2.9 5.8 5.8 5.8 5.8 5.8 5.5 6.1 PbO 5.5 5.8 5.8 5.8 5.8 5.8 5.5 6.0 5.08

MgO 4.4 BaO 14.9 7.8 27.9

BeO 5.0 Sb O 1.0 1.0 1.0 1.0 1.0 1.0 1.0

With respect to glass-ceramic articles, the above obrutile fibers enveloped within a residual glassy matrix, jective can be achieved by subjecting the glass bodies the total crystallization that can conceivably be present containing the desired large fibers of single rutile crysin such article is the full TiO content. However, the tals to a further heat treatment at temperatures ranging 25 percent fibers commonly developed within the between about 600850C. since the rate of growth of body frequently produce deep opacity therein and can the crystals is a function of temperature, longer expoi rt a three-fold improvement in mechanical sure times will be required at the cooler extreme of the strength d toughness t th it crystallization range. However, experience has demonwh a 1 i i l i d i d h fib strated that at least about one-half hour w1ll be decontaining glass b d i exposed'to temperatures b "landed to Promote a Substantial growth of crystalstween about 600-850C. for periods of time in excess Much f g times t be employed, e 1 "9 24 of about one-half hour. At all exposure temperatures, hours, with no deleter1ous effect. Nevertheless, times X ray diffraction analyses have evidenced the presence between aboflt hours h P deemed preferrfad 40 of fine-grained rutile crystallization. At temperatures from a pract1cal point of view w1th regard to the denslty b E10W about 700C anatase (another crystalline form and character of crystal growth. In general, the crystals of Tioz) has also been observed whereas, at higher will e i g i p is an time-gamed temperatures, all the TiO crystals have commonly 658 t an a out mlcrons m lame been identified as rutile. Crystal phases other than TiO At temperatures below about 600 C., the rate of have been observed, depending upon the glass compocrystal growth becomes so slow as to be commerc1ally o sition. However, wlthin the preferred glass compos1t1on unattractlve. Temperatures above about 850 C. can be range, the pr1nc1pal secondary phase identified has ut1l1zed but undes1rable interreactlon with the large ru- 0 tile fibers is hazarded been AI B O after a three-hour exposure to 650 C. or 760C. Determinations of crystal content through It frequently Customary to Cool the ff f 5O X-ray diffraction have indicated the presence of rutile glass bodlesto P temperatflrie to Perm1t mspectlon in the glass-ceramic article in an amount 4-5 times that of glass q y Prlor to crystanlzmg the y to a glass observed in the parent fiber-containing glass. ceramic. However, that practice is not required to w claim; achieve the desired subsequent fine-grained crystalliza- A composite article consisting essentially of long tlon. It 15 only necessary to cool the glass to atempera- Single crystal rutile fibers of high aspect ratios com ture below the deformation temperature thereof and tamed Within a quaternary RO TiO2 A12O3 B2O3 glass expose to a temperature wlthm the 600 850 range' matrix, said article exhibiting chemical durability and In general, the starting glass batches will be limited having an overall composition consisting essentially, by to the quaternary system RO-TiO -Al O -B O within weight on the oxide basis, of about 45-65% B 0 the ranges delineated above since additions of other 530% A1 0 530% TiO and 3-30% RO, wherein components hazard the inhibition of TiO crystalliza- RO consists of an alkaline earth metal oxide selected tion and/or the development of unwanted, secondary from the group MgO, CaO, SrO, and BaO. crystallization. However, minor amounts of compatible 2. A composite article according to claim 1 wherein metal oxides can be tolerated with the total of all such said fibers range up to about 0.5 inch in length and 0.004 inch in width, with aspect ratios up to about 200:].

3. A composite article according to claim 2 wherein said fibers range between about 0.1-0.4 inch in length 5 6 and 0001-0004 inch in width, with aspect ratios be- 5. A composite article according to claim 1 wherein tween about 50400: said composition consists essentially of about 50-65% 4. A composite article according to claim 1 wherein said composition is essentially free from SiO P F, B203 -25% M203 5 20% T102 and 345% Lizo, N320, and K20. 5 

1. A COMPOSITE ARTICLE CONSISTING ESSENTIALLY OF LONG SINGLE CRYSTAL RUTILE FIBERS OF HIGH ASPECT RATIONS CONTAINED WITHIN A QUATERNARY RO-TIO2O3-B2O3 GLASS MATRIX, SAID ARTICLE EXHIBITING CHEMICAL DURABILITY AND HAVING AN OVERALL COMPOSITION CONSISTING ESSENTIALLY, BY WEIGHT ON THE OXIDE BASIC, OF ABOUT 45-65% B2O3, 5-30% AL2O3, 5-30% TIO2, AND 3-30% RO, WHEREIN RO CONSISTS OF AN ALKALINE EARTH METAL OXIDE SELECTED FROM THE GROUP MGO, CAO, SRO, AND BAO.
 2. A composite article according to claim 1 wherein said fibers range up to about 0.5 inch in length and 0.004 inch in width, with aspect ratios up to about 200:1.
 3. A composite article according to claim 2 wherein said fibers range between about 0.1-0.4 inch in length and 0.001-0.004 inch in width, with aspect ratios between about 50-100:1.
 4. A composite article according to claim 1 wherein said composition is essentially free from SiO2, P2O5, F, Li2O, Na2O, and K2O.
 5. A composite article according to claim 1 wherein said composition consists essentially of about 50-65% B2O3, 15-25% Al2O3, 5-20% TiO2, and 3-15% RO. 